Fenrir or fenris is a name of a wolf that appear in Norse Mythology, which as a Wolf that predicted will kill every God that he meet in God Annihilation (Ragnarok). He is son of God of Evil (Loki) with a Frost Giantess (Angrboda). Because the god afraid to him, even Odin didn't dare to face him directly, he then kidnapped and taken to the Asgard. In Asgard sealid with Gleipnir ( A rope that made by the Dwarf from farious strong materials), maybe the rope looks very brittle but the rope was absolutely strong to seal Fenrir.
But to seal Fenrir, a God named Tyr must sacrifice his hand to seal this wolf, to prove that the seal wasn't dangerous for Fenrir. He sealed on a stone and his mouth also sealed with a piece of sword to make sure that he wouldn't bit anyone else. For thousand of years only Tir who dare and allowed to get near him. But Fenrir finally free at Ragnarok and kill a lots of God, and his first victim was Odin, the lord of the God. In Final Fantasy Series he appear as one of Eidolons in Final Fantasy IX, also become one of the summon and also mobs in Final Fantasy XI.